Microhardness under strain. 2. Microhardness behavior during stress-induced polymorphic transition in block copolymers of poly(butylene terephthalate)

Abstract

In addition to homopoly(butylene terephthalate) (homo-PBT), the microhardness technique was applied to its multiblock copolymers for examination of the stress-induced polymorphic transition. Drawn and annealed at 170°C for 6 h with fixed ends in a vacuum, bristles of poly(ether ester) (PEE) having PBT as hard segments and poly(ethylene oxide) (PEO) with molecular weight 1000 as soft segments (PBT/PEO = 57/43 wt%) were characterized with respect to their microhardness H at various stages of tensile deformation. A sharp H decrease (by 20%) in a narrow deformation interval (2—4%) due to the stress-induced α-β polymorphic transition is observed followed by an increase and decrease of H. The transition is registered at much higher relative tensile deformations (between 25% and 30%) compared with the case of homo-PBT. This is explained by the presence of a very soft amorphous phase of PEO, which deforms first. The same phase, distinguished by low viscosity in which the PBT crystallites are “floating,” is the reason for the very low H values measured (between 25 and 35 MPa, depending on the amount of α-and β-modifications). The stress-induced polymorphic transition in copolymers of PBT with PEO is demonstrated for the first time, also supporting the recently proposed concept of the microhardness depression effect.